The snail-trematode system with its multiple host-parasite interactions represents an advantageous model for the study of complex patterns of parasite behavior. Our continuing investigations will attempt to discern the various expressions and underlying factors controlling the eclimostome-schistosome interactions in vector snails. These background studies are tied to mass screening of snails from areas of human schistosomiasis to enable us to select candidate species and judge their potential as schistosome-destroying trematodes to serve as agents of biological control in areas of human disease. With critically controlled dosage and infection intervals, appropriate pairs of interacting trematode species, and observational methods developed in this laboratory, along with standard histological and EM techniques, we will study direct and indirect-antagonism, synergism, interference, and snail immunity to reinfection. The degree, timing of onset and developmental modification of trematode dominance is of special interest in the study of direct or predatory interaction. Indirect antagonism causes developmental delay in rival trematodes. This will be examined in terms of direct toxicity (cell lysis or dedifferentiation), interference with host nutrition or other expressions of competition. Host effects include synergism (reduced resistance caused by a prior heterologous infection), and immunity (heightened or absolute resistance to a homologous challenge infection). Interference (modification of dominance in a 3-way parasite system) will also be investigated. The degree of specificity of two recent immunological findings are of particular interest: parasite mimicry (utilization of host antigens), and antigen-binding capacity in the hemolymph of infected snails.